Understanding the structure and properties of geological formations is important for a wide variety of applications in well and reservoir management, monitoring, and remediation. Fluid sensing devices can detect certain properties of fluids to aid in attaining this understanding. Ongoing efforts are directed to development and use of fluid sensing devices that can monitor all fluids that are in place in a wellbore, including those put in place by operators, and those indicating the existence of an undesirable condition, such as contamination or invasion.